1. Field of the Invention
The present invention relates to corrosion inhibition in acidic, aqueous media, and more particularly to inhibition of corrosion of ferrous surfaces in refinery overhead streams and distillation towers.
2. Description of the Prior Art
A solution has long been sought to the common and troublesome problem of corrosion of ferrous surfaces in oil refinery overhead streams (in particular, of the crude distillation unit and vacuum distillation tower) and other distillation towers. In particular, it has been difficult to solve the problem because such streams are highly acidic, typically having a pH of from less than 1 to about 3, and are maintained at temperatures exceeding about 200.degree. F. (93.degree. C.). By contrast, conventional corrosion inhibitors generally are employed in environments that are characterized by far less severe conditions. For example, corrosion inhibitors employed in oil field pipelines generally are not considered satisfactory corrosion inhibitors for refinery overhead streams and distillation towers, first because the disparate nature of the oil field pipeline and refinery/distillation arts results in a failure to consider application of corrosion inhibitors from one art to another art, but also because oil field pipelines ordinarily are not strongly acidic (rarely, if ever, having a pH below about 4) and are at generally ambient temperatures. Thus, oil field corrosion inhibitors are not recognized as effective in highly acidic, high temperature conditions, which conditions themselves increase corrosion rates dramatically.
Accordingly, whereas the refinery and distillation streams include the strong acid, HCl, with which the corrosion therein is associated, and are maintained at a temperature of at least about 200.degree. F. (93.degree. C.), and more commonly as high as 300.degree. F. (149.degree. C.) or more, oil field pipeline corrosion is associated with weak acids due to the presence of hydrogen sulfide and carbon dioxide and typical pipeline temperatures are under 100.degree. F. (38.degree. C.).
Because corrosion inhibitors have not been found to be satisfactory under the low pH, high temperature conditions of refinery overhead streams and distillation towers, it has been common practice to attempt to resolve at least the acidity problem by neutralizing the stream by addition of ammonia or certain organic amines, such as ethylene diamine, to raise the pH above 4 (generally to about 6) before addition of the corrosion inhibitor. This technique has been found to be unsatisfactory not only because of the extra treatment step and extra additive required, but also because the amines added to the stream tend to form corrosive HCl salts, which tend to exacerbate the problem and to corrode. Yet, commercial processes which do not incorporate ammonia or an organic amine are virtually unknown. Thus, efforts to find suitable corrosion inhibitors for such applications typically have not produced entirely satisfactory results.
Accordingly, while U.S. Pat. Nos. 4,332,967 and 4,393,026, both to Thompson et al., mention that the particular compounds disclosed therein might be applicable to refineries or distillation towers, corrosion inhibitors for oil field pipelines are not recognized to be applicable generally to refinery overhead streams, especially without first neutralizing the HCl in such streams. Thompson et al. also mentions (at col. 20, lines 29-33 of '967 and col. 20, lines 4-8 of '026) that the corrosion inhibitors described therein are effective in systems of "high temperature, high pressure and high acidity, particularly in deep wells, and most particularly in deep gas wells." However, the acidity of such wells is recognized not to be below about pH 3.5, generally not below pH 4.
Thus, Thompson et al. do not suggest that the compositions described therein would be effective at lower pH's (as found in refinery overheads), or that their use in refineries would be in a manner other than the standard, conventional technique, which calls for addition of ammonia or amine to increase the pH above 4 (with the problems connected therewith). And more generally, conventional corrosion inhibitors have been found to be either ineffective or susceptible to entering into undesirable side reactions in the highly acidic conditions of refinery overheads. Moreover, while combinations of neutralizers, filming inhibitors, and water washes with water soluble filming inhibitors have been employed in overheads, no satisfactory solution to internal tower corrosion has been found.
Thus, corrosion inhibitors that are effective in the low pH, high temperature conditions of refinery overhead streams without the need for neutralizing the HCl in such streams are needed.